X-ray emission of the Be star/pulsar system PSR B1259−63
at non-periastron phases
Yuki ONO,1 Yoichi YATSU,1 Mikio MORII,2Nobuyuki KAWAI1 and the MAXI team
1Department of Physics, Tokyo Institute of Technology, 2-12-1, Ohokayama, Meguro, Tokyo, Japan
2
The Institute of Statistical Mathematics, 10-3-D414, Midoricho, Tachikawa, Tokyo, Japan
E-mail(YO): onoy@hp.phys.titech.ac.jp
Abstract

We report on the MAXI observation of the field of PSR B1259−63. X-ray flares have been observedat orbital phases far from the periastron. However, IGR J13020−6359 is located at 0.16 deg from PSR
B1259−63, making it diﬃcult to identify the sources of the flares. In this study, we fit the X-ray images
with the Point Spread Function of MAXI GSC to determine the position of the X-ray flares accurately.
As a result, we can distinguish the flares of PSR B1259−63 from those of IGR J13020−6359; three flaresout of five flares are likely to be from PSR B1259−63. We suggest that its Be star companion temporarily
releases a large amount of gas in directions other than the circumstellar disk, resulting in increases of thestellar wind density around the pulsar at phases far from the periastron.
Key words: pulsars: individual(PSR B1259−63) — X-rays: binaries

1. Introduction
PSR B1259−63/SS 2883 is a binary system in which apulsar orbits around a Be star with a long orbital period
of about 3.4 year. It is argued that the system is nonaccreting and a shock is formed between the pulsar wind
and the Be star wind, resulting in particle accelerationand non-thermal high energy emission near periastronup to TeV gamma-ray bands (Tavani et al. 1997).
2. Observation
Figure 1 is the X-ray light curves of the field of PSR
B1259−63. According to Figure 1, X-ray flares havebeen observed at orbital phases far from the periastron.
However, the resolution of MAXI GSC is typically on
the order of 1.5 deg, while a nearby transient source IGR
J13020−6359 is located at 0.16 deg from PSR B1259−63,
making it diﬃcult to identify the sources of the flares.
The source of Flare 1 is identified as IGR J13020−6359
by Swift/XRT observation (Kong et al. 2010) and thesource of Flare 2−5 has not been identified due to lackof concurrent XRT observation.
3. Analysis and results
In order to identify the source of the flares, we determined the position of the X-ray flares with using the PSF
fit tools (Morii et al. 2016). We formed a PSF modelFig. 1. MAXI/GSC and Swift/BAT light curves of the field of PSR
B1259−63. The upper panel shows the MAXI GSC light curve
and the lower panel shows the Swift BAT light curve. Periastrons
and flares are highlighted with shades.

consisting of a flare source, catalogued X-ray sources
excluding PSR B1259−63 and IGR J13020−6359, andbackground photons on a detector coordinate of MAXIGSC. We then allowed the position of the flare source
to move and determined its position to maximize thelikelihood function. Figure 2 is the results of the PSF
fit determinating the position of the X-ray flares. Fig-Flare ID
Flare 2
Flare 3
Flare 4
Flare 5orbital phase
ϕ0.085
0.641
0.962
0.148

Flux (1-20 keV)
×10−10 erg cm−2 s−1
2.81+0.51−0.60
4.76+0.50−0.71
7.41+0.50−2.31
3.83+0.21−0.73

L1−20keV×1035 [erg cm−2 s−1 ]1.77+0.32−0.37
3.00+0.32−0.45
4.67+0.32−1.46
2.42+0.13−0.46

Table 1. A list of the flux and the luminosity of the observed flares. We assumed that PSR B1259−63 is located at a distance of 2.3 kpc.

Fig. 3. The calculation results of the X-ray luminosity.

ξ0.03
0.05
0.08
0.1
Fig. 2. The calculated position of the flares with using the PSF fit
tool. The cross marks show tha calculated position. The dashedlines show that the statistical 90 ％ confidence level. The solidlines show that the 90 ％ confidence level taking the systematic
error of MAXI GSC into account.

ure 2 shows that the source of three flares out of four
flares are consistent with PSR B1259−63 (Flare 2, 3, 5).
The position of the flare 4 is near IGR J13020−6359 but
contains the coordinates of both IGR J13020−6359 and
PSR B1259−63. Therefore, we included the flare 4 indiscussion.
4. Discussion
We assumed that synchrotron cooling dominates theemission of these flares from the shocked wind and thatthe shocked electron pairs have a power-law distribution
N (γ) = Kγ −p (γmin < γ < γmax ). We then calculated
the X-ray luminosity emitted from the shock-accelerated
electron/positron (Murata et al. 2003). The parameters
we used for the calculations are the minimum Lorentz
factor γmin , the power law index p, the conversion eﬃciency from the pulsar spin-down luminosity to the relativistic electron/positron pairs ϵa , and the ratio of theshock distance from the pulsar to the binary separation

Flare 2
46.7
16.1
5.92
3.63

Flare 3
161.5
55.3
20.4
12.1

Flare 4
12.0
4.13
1.51
0.929

Flare 5
108.3
37.6
13.6
8.23

Table 2. The mass loss rate of the sphericlly symmetric Be staroutflow Ṁ with each ξ. The mass loss rates Ṁ in the table isgiven in units of 10−6 M⊙ /yr.

ξ. We found that the observed flux can be explained
when ϵa is ∼0.8 and ξ is <0.1 (Figure 3).
We suggest that the Be star temporarily releases alarge amount of gas in directions other than the circumstellar disk, resulting in increases of the stellar wind density around the pulsar at phases far from the periastron.
When we assume that the Be star wind is sphericallysymmetric, such a small shock distance is possible whena mass loss rate of the Be star Ṁ is 10−4 − 10−6 M⊙ /yr(Table 2).
References
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